Miguel did a very nice job explaining how this move frees up $6+ million in cap space in 2014, 2015 and 2016, and how unused cap space rolls forward to the following year. The Mankins trade frees up valuable dollars to re-sign guys like Revis and McCourty. These funds being available is a central part of this trade.
At 32-years-old, and probably in the last multi-year deal of his NFL career, Mankins has every right to say no to renegotiating a deal he worked hard to get. He's already collected his signing bonus, and he has every reason to opt for the uncertainty of being traded or cut to preserve as much of that money going forward as he can. The presumption is that the Buccaneers will keep him and are now responsible for everything but the bonus money, and the 2014 camp bonuses. Mankins will continue to get paid.
On the football side, the Patriots get a player that fills an empty role, and another 4th rounder in 2015. The Patriots also get younger along the Offensive Line. Without Mankins, the backups will get a huge number of snaps they would not normally get. The pressure, in my mind, is on Dave DeGuglielmo to coach 'em up and build a good unit. He's up to the task. He is one of the few coaches that came in from the outside with an impressive resume that included the Giants, Dolphins and Jets - all teams with good lines while he was there.
I must say, this trade has more to chew on than any since the Richard Seymour deal.